This invention relates to double-beam infrared spectrophotometers of electrical direct ratio system, and more particularly, to such ratio system spectrophotometers capable of phase compensation by minimizing the noise resulting from phase deviations caused by rapid absorption in the frequency component detecting system.
As is well known in the art, the double beam spectrophotometers measure the transmittance of a sample by causing light to alternately enter the sample and a reference or standard material (or an empty cell), measuring the intensity of a sample beam that has passed the sample and a reference or standard beam that has passed the reference or standard material, and comparing the sample beam intensity with the reference beam intensity, with the resultant ratio giving the transmittance of the sample. The signal processing method in such double beam spectrophotometers, that is, the method of automatically outputting the transmittance of the sample is generally classified into the optical null balance system widely used in conventional infrared spectrophotometers, the electrical direct ratio system, and the automatic gain control system, of which the latter two were recently developed.
The optical null system measures the transmission of a sample by alternately switching a sample beam and a reference beam attenuated by a mechanical beam attenuator, taking out an AC signal having an amplitude proportional to the difference (I.sub.0 -I) between the reference beam intensity I.sub.0 and the sample beam intensity I, using this signal as an error signal in a closed loop, thereby automatically adjusting the mechanical attenuator associated with the reference beam such that the value of the difference (I.sub.0 -I) may always become zero. When the attenuator is adjusted in this way, the magnitude of beam attenuation itself, that is, the distance of movement in the attenuator is proportional to the transmittance so that changes in the transmittance of a sample can be recorded by recording the movement of the attenuator.
The above mentioned optical null system is best with respect to percent utilization of signals and stability of measurement, but has several drawbacks described below. First of all, since the accuracy of transmittance largely depends on the mechanical accuracy of the attenuator itself as well as the associated drive system, the spectrophotometer is difficult to exhibit highly accurate and stable performance. In connection with this, due to fluctuations in rotation of a servo motor for driving a wedge-shaped stop commonly used in the attenuator, errors in linearity of a potentiometer for detecting the position of the wedge-shaped stop, and other factors, the distance of movement of the stop is not always proportional to the magnitude of attenuation, often resulting in low accuracy of transmittance measurement. Further, the inclusion of the optical system in the servo loop results in a complicated and expensive apparatus which handles signals in a complicated way and has poor response. In the case of a sample having high absorbance, the sample beam intensity approximates to zero, and accordingly, the reference beam intensity is also attenuated to a level near zero, resulting in reduced loop gain and reduced reliability. In addition, the attenuator itself is reduced in accuracy when the degree of attenuation is very high, that is, when the sample has very high absorbance. These undesirably causes a substantial reduction in accuracy of measurement of a high absorbance sample.
The automatic gain control system which does not use a beam attenuator is a system in which time division is made by beam path switching means such as a sector mirror to allocate fractions to sample and reference beams and dark state when both sample and reference beams are interrupted, and the gain of signal processing means, for example, the gain of a detector or an amplifier is automatically controlled such that the reference beam intensity may always be at a constant level. In this system, an output of the amplifier corresponding to a sample beam directly corresponds to the ratio of sample beam intensity to reference beam intensity, that is, the transmittance of the sample. The transmittance is then directly available simply by sample holding an output of the amplifier corresponding to a sample beam. This system uses a gain controllable detector, for example, photomultiplier as the detector. When the detector gain is controlled in a feedback manner, all electric signal systems are contained in this loop so that the linearity and stability of detector and amplifier have no influence on measurement, insuring high accuracy in measurement. Further, because of the absence of a beam attenuator and a mechanical servo system and the possible setting of absolute zero, this system has eliminated most of the drawbacks of the optical null system. However, infrared spectrophotometers carring out spectral analysis in the infrared region must use thermal detectors such as thermocouples. The thermal detectors are difficult to control their sensitivity and thus incompatible with the automatic gain control system. A spectrophotometer may be constructed using a thermal detector such that the thermal detector is connected to control the gain of an amplifier. However, since the thermal detector has a considerably slower speed of response and a considerably larger time constant than a photomultiplier detector used in visible-to-ultraviolet spectrophotometers, and the output waveform of the thermal detector does not correspond to the waveform representative of changes of the beam which has passed the beam path switching means, it is difficult to devise a practical automatic gain control system using a thermal detector.
The electrical direct ratio system does not use a beam attenuator and is a system in which an output signal of a photodetector containing in admixture components corresponding to the intensities of sample and reference beams is amplified by a common amplifier before the individual components are electrically separated and the ratio of the individual components is electrically computed. This system is generally subdivided into two systems, frequency component detection system and phase discrimination system, depending on how to separate and take out components representative of sample and reference beam intensities from the amplifier output. In either case, separation of signal components can be carried out even when thermal detectors such as thermocouples are used. Consequently, this system is adaptable to infrared spectrophotometers.
Among prior art spectrophotometers of the electrical direct ratio system, specifically the frequency component detection system is one disclosed in Japanese Patent Application Kokai No. SHO 52-10790 (published on Jan. 27, 1977). This spectrophotometer uses as beam path switching means for interrupting and switching beam paths for sample and reference beams, a sector mirror capable of alternately discontinuing the sample and reference beams at a given frequency f and a chopper capable of discontinuing the sample and reference beams from the sector mirror at a frequency 2f twice the frequency f of the sector mirror. Since a component having frequency f in an output signal of the detector corresponds to the difference (I.sub.0 -I) between the reference beam intensity I.sub.0 and the sample beam intensity I, and a component having frequency 2f corresponds to the sum (I.sub.0 +I) of the reference beam intensity I.sub.0 and the sample beam intensity I, an output proportional to I.sub.0 is obtained by adding the components having frequencies f and 2f and another output proportional to I is obtained by subtracting the one component from the other component. The value of I/I.sub.0 may be obtained by computing the ratio of these outputs.
Also known as the electrical direct ratio determining double-beam spectrophotometers based on the frequency component detection system is one wherein a chopper having a chopping frequency f is located in the sample beam path, another chopper having a chopping frequency 2f is located in the reference beam path, and a half mirror is provided so as to guide the sample and reference beams into a common beam path to enter a photodetector through a monochromator. With this arrangement, that component of the output signal of the photodetector which has frequency f is proportional to the intensity of the sample beam intensity and that component of the photodetector output which has frequency 2f is proportional to the reference beam intensity.
In the general practice of spectrophotometers, the monochromator is operated so as to sequentially change the wavelength of detection, that is, so-called wavelength scanning is carried out, thereby determining changes in the absorbance of the sample in response to such wavelength changes. However, in the case of infrared spectrophotometers, as wavelength scanning is carried out in the presence of atmospheric H.sub.2 O and CO.sub.2 in the beam paths, it has been observed that specific absorption by H.sub.2 O and CO.sub.2 rapidly changes in a certain wavelength region. Such rapid changes of absorption due to H.sub.2 O and CO.sub.2 will result in rapid changes of the incident energy to the photodetector, which cause the output waveform of the photodetector to be deformed, resulting in phase deviations. In general, such changes of absorption due to H.sub.2 O and CO.sub.2 in air will occur to a substantially equal extent in the sample and reference beam paths, and the spectrophotometer of the electrical direct ratio system is designed to determine the absorbance of a sample from the direct ratio of the sample beam intensity to the reference beam intensity. At first sight, it is considered that the absorption by H.sub.2 O and CO.sub.2 in the sample beam path and the absorption by H.sub.2 O and CO.sub.2 in the reference beam path are mutually cancelled in computing the direct ratio, giving no influence on the output data indicative of the absorbance of the sample. Nevertheless, in the frequency component detection system designed to measure the absorbance of a sample by detecting mutually different frequency components and determining the ratio of them, the above-mentioned rapid absorption changes affect to different extents at different frequencies, and thus changes of absorption due to H.sub.2 O and CO.sub.2 are not actually cancelled from a measurement of the absorbance of the sample, introducing measurement errors in the output data. More specifically, rapid changes of absorption due to H.sub.2 O and CO.sub.2, that is, rapid changes of the incident energy to the photodetector induce phase deviations in the output signal of the photodetector. Since a filter having sensitive phase characteristics is used for the frequency discrimination of the output signal of the photodetector, noise is introduced into the output of the filter to different extents at different frequencies. As a result, a measurement of the absorbance obtained by computing the frequency components does not represent a correct sample-to-reference ratio.
It is, therefore, an object of the present invention to provide a spectrophotometer which has eliminated the above-described drawbacks of both the optical null balance system and the frequency component detection system.
It is another object of the present invention to provide a spectrophotometer based on the frequency component detection system and the electrical direct ratio system in which the occurrence of errors due to phase deviations encountered where absorption is rapidly changed by H.sub.2 O and CO.sub.2 in air is minimized.
It is a further object of the present invention to provide electrical processing means for compensating for the above-described phase deviations because the phase deviations are proportional to changes of the incident radiation energy to a photodetector.